If you have a residence order in your favour your mother would automatically become guardian on your death. If there is no residence order she would only become guardian when everyone with PR has died. If there is a dispute about where the children live after you have died the courts will resolve the matter by deciding what is in the children's best interests.
You can say who you want to look after the children in the event of your death, but it isn't legally binding. If the father wishes to take care of them and there are no reasons for him not to (abuse, drugs) then he will almost certainly achieve that.
He has been their father and your husband for some years? I think that would strengthen his case too. An absent biological father would probably be different.
It's something that court-ordered NRP should be aware of though, and possibly seek urgent legal advice, should their DCs RP die unexpectedly.
My DHs DS could be put on a plane by family members to live with a guardian abroad before DH is even made aware of his exW death - yes, it could be thrashed out in court, but by then, his DS would be settled with a new family and very little chance of regular contact.
Frogbyanothername - If your husband has PR it is a criminal offence for other family members to take his son out of the country without his consent. If the country concerned is one that follows the Hague Convention there should be no problem getting the child returned to the UK.
we have no resident orders about the children but I was told on the divorce that I can have a Letter of Wishes(which I do and a will) but their father can have them no matter what other wish I express as he is the father. That seems to make sense to me - he is the other parent. I presume a mother who was separated and the children lived with the father would want the children if the father died so why not the other way round?
Just for clarity, while the mother is alive the position is slightly different. If the mother has a residence order in place she can take her son out of the country (or give permission for someone else to take him out of the country) for up to one month without needing your husband's consent. But neither she nor anyone else can take him out of the country for longer than one month.
What concerns DH is the fact that (if we understand correctly) the testimonial guardianship put in place by his DS Mum would come into effect immediately upon her death so the guardian can act with PR independently of DH - without DH even knowing. The likelyhood is that the guardian nominated by DH exW lives abroad or many hundreds of miles away in the UK, and so there is a very real chance of his DS being removed (even illegally if abroad) to live there. Once hes there, any court action instigated by DH will primarily consider his DS welfare - and it's questionable whether it would be in his DS interests to uproot him again to return to live with us and of course, contact would be significantly restricted by distance. The DC in question is 10 at the moment - I'm guessing that as he gets older, his wishes and feelings would carry increased weight in court.
Once hes there, any court action instigated by DH will primarily consider his DS welfare
No. Whether or not returning him to this country was in DS's best interests wouldn't even be on the court's radar. His DS will have been illegally removed from the country and a criminal offence would have been committed (child abduction). The first priority would be to get him back regardless of how long he had been absent. There would be absolutely no question of allowing his abductors to benefit from their crime by keeping him abroad.